The present disclosure relates generally to display devices and methods for making and driving the display devices, and more particularly, to display devices having light emitting elements.
Lighting emitting diodes (LEDs), such as organic LEDs (OLEDs), completely solve the problems of narrow viewing angle and light loss because of their self-illuminating nature and thus, are considered as the most promising technology for next-generation display devices. Similar to liquid crystal displays (LCDs), the driving circuits of OLED display devices are fabricated on glass substrates using photolithography techniques. Because OLEDs are current-driven components, which require a relatively higher current level, low temperature polysilicon (LTPS) is usually used as the channel material to satisfy the channel mobility requirement. As the light emitting materials of OLEDs are organic materials, the patterns of the OLEDs are usually defined by evaporation techniques using shadow masks.
FIG. 1 illustrates a side view of a conventional OLED 100 fabricated on a glass substrate 102. An insulating layer 104 is formed on the glass substrate 102. The OLED 100 includes an anode 106 and a cathode 108. From the anode 106 to the cathode 108, a hole injection layer 110, an organic light emitting layer 112, and an electron transport layer 114 are formed in this order. Driving circuit 116 is also formed on the glass substrate 102, which includes driving and switching thin film transistors (TFTs), wires, and vertical interconnect accesses (vias). Depending on the materials used in the organic light emitting layer 112, light of different colors (wavelengths) may be emitted through a cover glass 118.
Multiple OLEDs (sub-pixels) may constitute a single pixel on the display. As shown in FIG. 2, a pixel 200 includes two OLEDs 202, 204, each of which is formed on the glass substrate 102 and driven by corresponding driving circuit 116. The cathode 108 is a common cathode for both OLEDs 202, 204. Similarly, each of the insulating layer 104, hole injection layer 110, and electron transport layer 114 is a common layer for both OLEDs 202, 204. However, each organic light emitting layer 112 needs to be individually patterned by evaporation techniques using shadow masks to fit the shape and size of each sub-pixel. In FIG. 3, a pixel 300 includes three OLEDs 302, 304, 306 formed on the same plane as sub-pixels, which emit lights of red, green, and blue from their corresponding organic light emitting layers 308, 310, 312, respectively. As noted above, the common anode 106 and common cathode 108 are formed for all the OLEDs 302, 304, 306, while the organic light emitting layers 308, 310, 312 are individually patterned for each sub-pixel.
Due to the process accuracy for patterning organic materials using shadow masks, the minimum size of each organic light emitting layer is limited. Moreover, as all the OLEDs are formed in the same plane in conventional devices as shown in FIGS. 1-3, sufficient spaces have to be maintained between adjacent sub-pixels to avoid overlapping of adjacent organic light emitting layers. Therefore, the resolution of the conventional OLED display devices is limited by the process accuracy of the organic light emitting layer and the planar structure of OLEDs.
Accordingly, there exists a need for improved display devices and method for making and driving the display devices to solve the above-mentioned problems.